This research is focused on the molecular mechanisms that govern whether a primary breast cancer will or will not become aggressive, metastatic and thus life threatening. At present, the ability to predict such aggressiveness is imperfect, in that there is great inter-individual variability in the behavior of a group of tumors that are all classified together in one of the major subgroups of breast cancer and thus predicted to share a common prognosis. At present, this inability to generate accurate predictions of the future behavior of individual breast cancers leads to aggressive treatment of the great majority of diagnosed tumors, when only a minority are destined to become life threatening. The research describes three major determinants of malignant progression of breast cancer cells and how they conspire to generate aggressive behavior. These are (i) the ability of carcinoma cells to release pro-inflammatory signals; (ii) the reciprocal responses of nearby mesenchymal stem cells within the stroma of tumors to these carcinoma-derived signals, resulting secondarily in the release of signals that have the potential of inducing carcinoma cells to move from an epithelial(benign) to mesenchymal (malignant) state; and (iii) the propensity of the carcinoma cells to respond to these stroma-derived signals by undergoing this shift in differentiation state, thereby acquiring highly aggressive characteristics. The propensity of cancer cells to move from an epithelial/benign to a mesenchymal/malignant state appears to be governed by the state of the chromatin associated with the gene that encodes ZEB1, which functions as the key molecular governor of the epithelial vs. mesenchymal states. Examining the chromatin configuration of this gene- more specifically the covalent modifications of the histones associated with the promoter of this gene - holds the promise of revealing the proclivity of a breast cancer cell to activate its program of malignant conversion, often termed the epithelial-mesenchymal transition.